Congestion Avoidance and Control Van Jacobson, Michael J. Karels - PowerPoint PPT Presentation

Congestion Avoidance and Control Van Jacobson, Michael J. Karels Presented by Naveen Cherukuri

Overview  Introduction  Getting to equilibrium  Conservation at equilibrium  Adapting to the path  Experimental evaluation  Future work

Introduction  Explosive growth of Networks  Gateway- Local buffer overflows  Cause lies in transport protocol implementation  First Congestion collapse observed  October 1986, LBL to UCB  Data throughput dropped from 32Kbps to 40bps

Packet Conservation  Packet Conservation Principle 5 algorithms developed based on this  Round-trip-time variance estimation 1. Exponential retransmit timer backoff 2. Slow-start 3. More aggressive receiver ack policy 4. Dynamic window sizing on congestion 5.  Connection in equilibrium – Running stably with a full window of data in transit.  Conservative packet flow – Flow at equilibrium when “ A new packet isn‟t put into the network until an old packet leaves.

Failures  Three ways for “how packet conservation fails” 1)The connection doesn‟t get to equilibrium. 2)A sender injects a new packet before an old packet has exited. 3)The equilibrium can‟t be reached because of resource limits along the path.

Getting to Equilibrium: Slow-start  Connection Starting or Restarting  Self-Clocking Systems – adjust to bandwidth and delay variations

 To get data flowing acks to clock are required and to get acks there must be data flowing Problem – How to start Clock  Solution – Slow-start Algorithm to gradually increase the  amount of data-in-transit

Slow-start Algorithm  Add a congestion window, cwnd , to the per-connection state.  When starting or restarting after a loss, set cwnd to one packet.  On each ack for new data, increase cwnd by one packet  When sending, send minimum of the receiver‟s advertised window and the cwnd .

 Analysis Takes Rlog 2 W time to get to equilibrium where R – Round  trip time and W – window size in packets.  Guarantees that a connection will source data at a rate at most twice the maximum possible on the path.

Conservation at equilibrium: Round-trip timing  Failure of sender‟s retransmit timer.  A good round trip time estimator – the core of the retransmit timer is the most important feature to survive heavy load.  TCP estimates mean round trip time using R = α R + (1- α )M M – round trip time measurement from the most recently acked data packet. α – Filter gain constant with suggested value of 0.9

 Retransmit timeout interval(rto) for the next packet sent is set to β R  β accounts for RTT variation  They argue that β should also be estimated to improve performance at low load as well as high load.  Retransmit timer – Exponential back-off

Adapting to the path: congestion avoidance Packets get lost for two reasons  They are damaged in-transit 1. Network is congested. 2. Congestion avoidance strategy  Network must signal to transport endpoints that congestion is  occurring. The endpoints must decrease or increase utilization based on whether  the signal is received or not.

Congestion Window adjustment  Good candidate for „network is congested‟ signal?  Network load is measured by average queue length over fixed intervals of length near the roundtrip time.  L i = N ( uncongested network )  L i = N + γ L i-1 ( congested network )  γ indicates the amount of congestion  Source controls its loads by adjusting its window. W i = dW i-1 (d < 1) (On congestion)

Congestion Window adjustment  Network doesn‟t indicate if a connection is using less than its fair share.  Solution?  How about W i = bW i-1 1<b<=1/d ??  Rush hour effect  Hence W i = W i-1 + u (u << W max ) ( On no congestion)

Congestion Avoidance Algorithm  On any timeout, set cwnd to half the current window size (Multiplicative decrease)  On each ack for new data, increase cwnd by 1/cwnd (Additive increase)  When sending, send the minimum of the receiver‟s advertised window and cwnd.

Experimental evaluation

Future Work : The gateway side of congestion control  How to ensure “Fair sharing of the connection‟s capacity”?  Solution - Congestion detection algorithm at the gateways.  Misbehaving endpoints that do not consider the request to decrease the window size can be taken care of.

Thank you Questions and Suggestions ??